Possible Clues:

• The 'N' of TV's NPH
• It's often included with an R.S.V.P. card
• Enclosure for a slush pile, for short
• What an RSVP may come back in
• Request from an ed
• Sort of 57 Across
• Incentive for a return
• Enc. for a 32-Down
• Enclosure with a bus. ltr
• Enclosure with an RSVP